Not Applicable.
Not Applicable.
1. Field of Invention
The invention relates to an improvement in the sensitivity of a position sensitive gamma ray detector or gamma camera. More specifically, the invention describes a method of and an apparatus for producing useful position information from Compton scattered photons in a position sensitive gamma ray detector.
2. Description of the Related Art
Gamma-ray imaging is a useful tool in many areas of science, particularly in the biological and medical fields. For example, the radioisotope Technetium 99 m can be caused to be preferentially absorbed in tumors. The location of such a tumor in the body can be determined by forming an image of the 140 keV photons emitted by the decay of the isotope. Conventional lenses cannot focus such high-energy photons and typical x-ray detectors are relatively insensitive to them. Accordingly, the image is formed using a closely packed array of collimators. The image is read out using a position sensitive gamma-ray detector. When a photon is absorbed by the detector, its x- and y-position is determined and the corresponding x- and y-position in the image array is incremented by one. The image brightness is thus proportional to the number of photons absorbed by each pixel. The detector must discriminate between photons that have come directly from the source and those that have been randomly scattered. Because the scattered radiation is lower in energy than the unscattered radiation, the detector must have some degree of energy resolution. Such detectors are often scintillators such as NaI or semiconductors such as CdTe, CdZeTe, Hgl, or germanium.
The most commonly used gamma cameras are based on scintillators, but it has long been recognized that a semiconductor detector with better energy resolution might give better images. Because germanium gamma-ray detectors have good absorption efficiency and extremely good energy resolution, many attempts have been made to manufacture a position sensitive germanium detector for this application.
One of the earliest practical cameras was described by Kaufman, et al., IEEE Trans. Nucl. Sci, NS-22, 395, 1975. This camera used a planar germanium gamma-ray detector with collecting electrodes formed as an array of narrow strips. The strips on each side were orthogonal to those on the other side. Thus, the signal from the strips on one side gives the x-position and the signal from the strips on the other side gives the y-position. If the strips are of equal width, then the effective pixel is a square with sides equal to the strip width.
Good position resolution requires the pixels to be small, usually one to three millimeters on each side. If the arriving 140 keV photon is absorbed by a photoelectric event, then the signal will be a valid event for forming the image. However, even if the germanium detector is thick enough to interact with most of the arriving 140 keV photons, the small effective detector area means that photons interacting by Compton scattering, even through a small angle, will be lost to that particular pixel. The signal produced will be smaller than the original photon energy, indistinguishable from those scattered in the body, and will thus be lost to the measurement. Because approximately one-half of the interacting photons at 140 keV are Compton scattered, the sensitivity of the detector is reduced.
Accordingly, there is a need for a system that is capable of measuring gamma rays that undergo Compton scattering.
Therefore, it is an object of this invention to provide a signal processing means and apparatus that greatly reduces the loss of sensitivity caused by Compton scattering in a gamma camera.
When photons are absorbed in matter by Compton scattering, the maximum amount of energy that can be deposited in a single scattering event is given by the following equation:       E    max    =            E      in        -          511              (                  2          +                      511                          E              in                                      )            
where Emax is the energy deposited in the detector in keV and Ein is the incident photon energy in keV. Equation 1 shows that, for energies up to 511/2 keV, the maximum energy deposited is less than one-half of the incident photon energy. For the 140 keV photon considered here, the maximum energy deposited in the detector for a single scattering event is about 50 keV. When one of these incoming photons is completely absorbed in two separate pixels in the detector, the sum of the pixel energies identifies the incoming photon as a valid unscattered event. The detector pixel that produces the lowest value of energy is the first interaction site and therefore the position that should be used to form the image. At energies higher than 511/2 some fraction of the incident photons will Compton scatter depositing more than half the incident photon energy. For energies up to several hundred keV that fraction will be small, thus statistically the pixel producing the smaller energy is still the pixel that should be used to form the image. Thus, by suitable processing of the signals, a large fraction of the otherwise unusable Compton scattered events are used to form the image, greatly improving the sensitivity of the camera. The figures and descriptions that follow will describe the method and apparatus required to correctly process such signals and achieve the increase in sensitivity.
The maximum increase in sensitivity and signal-to-background ratio is achieved by processing valid two pixel interactions by the preferred method described above. A somewhat simpler system can be used that gains much of the sensitivity of the preferred method. In the simpler system both pixels are used to form the image. One of the pixels is the correct image point and thus improves the sensitivity. The other pixel is randomly distributed over nearby image points and thus adds background counts. Because the background is distributed over a number of pixels and contributes no structure to the image it is clear that the overall image quality is improved. It is the intent of the present invention to include both methods, however, the preferred method is shown in the drawings.